本文主要是利用霍普金森撞擊試驗機及帽型(Hat Shaped)試件來探討AISI 4140合金在三種不同溫度及極高速剪切荷載下之絕熱剪切變形行為。實驗於低溫-100℃、室溫25℃及高溫300℃，且分別在應變速率為4.1x105s-1、5.1x105s-1及6.5x105s-1下進行，藉由實驗所得之巨觀機械性質與和微觀破壞機制，來探討以瞭解AISI 4140合金件在極高應變速率下的動態剪切特性及剪切帶微觀組織之變化。
實驗結果顯示，環境溫度、應變速率及應變量對AISI 4140合金的巨觀機械性質影響甚鉅，其塑流應力值隨著應變速率的增加及環境溫度的下降而上升；在達到最大值後，因為熱軟化之影響，塑流應力隨著應變量的增加而下降，且在環境溫度低的下降得更明顯。AISI 4140合金的應變速率敏感性係數隨應變速率上升及環境溫度的下降而增加；隨應變量增加而減少。熱活化體積則隨著應變速率之增加而下降，隨應變量增加而增加。隨著應變量的增加，加工硬化率及應變速率敏感性皆呈現下降的趨勢，而熱活化體積則會上升。
微觀組織經由金相觀察後，發現三種不同溫度下之AISI 4140合金的絕熱剪切帶均為相變態剪切帶，且剪切帶寬度隨應變速率增加而減小；且會隨著環境溫度的下降而減小。剪切帶中心的局部應變量最大，隨著離開剪切帶的距離增加局部應變量迅速減小。SEM破斷面分析發現三種不同溫度下之AISI 4140合金破壞特徵分別為韌窩、劈裂及節瘤組織形貌，隨著應變速率增加及環境溫度的增加，韌窩深度及劈裂密度亦隨之增加。藉由TEM觀察，可以發現剪切帶附近的波來鐵組織非常的明顯，且有部分晶粒受到極高速的剪切而改變流動方向。另外在剪切帶上發現到麻田散鐵組織以及極微小單晶粒的產生。

This study uses a compressive split-Hopkinson pressure bar with hat-shaped specimens to investigate adiabatic shear banding in AISI 4140 alloys tested at temperature of -100℃, 25℃ and 300℃ under extremely high strain rates ranging from 4.1×105s-1 to 6.5×105s-1. Optical microscopy (OM) and scanning electronic microscopy (SEM) techniques are used to analyze the shear band formation and the microstructural characteristics of the deformed specimens in order to establish the correlation between the adiabatic shear behaviour of the AISI 4140 alloys tested at temperature of -100℃, 25℃ and 300℃ and their respective mechanical properties.
The experimental results indicate that the shear properties of the AISI 4140 alloys are significantly dependent upon the applied strain rate, strain and temperature. For a constant temperature, the shear flow stress, shear strain, strain rate sensitivity, temperature rise and micro-hardness are all found to increase with increasing strain rate, while the activation volume decreases. Thermal softening takes place during the current adiabatic shear processes and has a significant effect on the dynamic shear mechanical properties of the AISI 4140 alloys. Thermal softening results in strain instability and strain localization, and eventually leads to fracturing of the adiabatic shear band. The OM observations reveal that adiabatic shear bands are formed in all the impacted specimens. The local shear strain decreases with increasing distance from the center of the shear band, while the width of the shear band decreases with increasing strain rate. The SEM results reveal that the fracture surfaces of the AISI 4140 alloys at temperature of -100℃, 25℃ and 300℃ are characterized by a dimple-like, cleavage and knobble structure, which indicate that the specimens fail in a ductile and brittle manner. In all case, the area of the cleavage region increases with increasing strain rate and, while the size of the dimple area reduces.
We could find that for the observation of TEM the structure of perlite around the shear band was very clear, and some of grains flowing direction were changed due to extrmely high shear force. And the growths of martinsite structure and ultra fine grain are found.

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